The closure of atrial septal defects by interventional catheterisation requires an accurate assessment of their morphology and anatomical relationships. This study evaluated transthoracic three-dimensional echocardiography for the selection of atrial septal defects accessible to an occlusive prosthesis. The transthoracic three-dimensional echocardiographic measurements of 17 patients (4 to 55 years) with ostium secundum atrial septal defects were compared with those of the surgeon in a prospective study. The maximal diameters of the defect, the height of the interatrial septum, the distances to the superior vena cava (postero-superior border) and inferior vena cava (postero-inferior border), to the coronary sinus and the tricuspid valve were measured as a reconstruction of the interatrial septum seen from the right atrium. The aortic border was measured from a three-dimensional view from the left atrium. Thirteen of the 17 investigations (76%) were exploitable. The diameters of the defect varied during the cardiac cycle (p = 0.0002). Ther correlations between the surgical and echocardiographic measurements varied from 0.82 for the maximal diameter to 0.6 for the postero-inferior limits. Three-dimensional echocardiography is capable of detecting all the contra-indications of an occlusive prosthesis: 2 inadequate postero-inferior and 1 inadequate aortic borders, 9 maximal diameters which were too large, 3 insufficiently high atrial septa, 1 double atrial septal defect. The coronary sinus was only visualised in 1 case. Transthoracic three-dimensional echocardiography is a non-invasive technique capable of improving the selection of atrial septal defects for interventional closure. The transoesophageal approach should be reserved for candidates selected by the transthoracic investigation for the detection of small structures (coronary sinus) and when the transthoracic window is poor.